Stellar memo encoding/decoding type-safe helpers

[truthixify]

Tier: S (½-1 day) Type: feature

Context

Stellar transactions carry memos (text/id/hash/return). Today consumers handle this manually with @stellar/stellar-sdk primitives. Provide typed helpers in the Wraith SDK.

Scope

• encodeMemo({ type, value }) returns proper Memo object
• decodeMemo(memo) returns { type, value } typed structure
• extractMemoFromTransaction(tx) convenience
• Validation: text ≤ 28 bytes, id is uint64, hash is 32 bytes
• Exported from stellar chain module

Acceptance criteria

• Helpers + tests
• Validation error messages
• Documentation page
• Demo follow-up for memo display in Activity

Files to start with

• New: sdk/src/chains/stellar/memo.ts